Strip loaded waveguide with low-index transition layer

ABSTRACT

A strip loaded waveguide comprises a slab and a strip, wherein the strip is separated from the slab. Nevertheless, a guiding region is provided for propagating an optical mode and this guiding region extends both within the strip and the slab. A layer of material having an index of refraction lower than that of the strip and the slab may be disposed between and separate the strip and the slab. In one embodiment, the slab comprises a crystalline silicon, the strip comprises polysilicon or crystalline silicon, and the layer of material therebetween comprises silicon dioxide. Such waveguides may be formed on the same substrate with transistors. These waveguides may also be electrically biased to alter the index of refraction and/or absorption of the waveguide.

PRIORITY APPLICATION

[0001] This application claims priority under 35 U.S.C. §119(e) fromU.S. Provisional Patent Application Serial No. 60/318,456, entitled“Strip Loaded Waveguide with Low-Index Transition Layer” and filed Sep.10, 2001 as well as U.S. Provisional Patent Application Serial No.60/318,445 entitled “SOI Waveguide with Polysilicon Gate” and filed Sep.10, 2001, both of which are hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is directed to waveguides, and moreparticularly, to waveguides formed on a substrate.

[0004] 2. Description of the Related Art

[0005] Light offers many advantages when used as a medium forpropagating information, the foremost of which are increased speed andbandwidth. In comparison with electrical signals, signals transmittedoptically can be switched and modulated faster and can include an evengreater number of separate channels multiplexed together. Accordingly,lightwave transmission along optical fibers is widespread in thetelecommunications industry. In an exemplary fiber optic communicationsystem, a beam of light may be emitted from a laser diode and modulatedusing an electro-optical modulator that is driven by an electricalsignal. This electrical signal may correspond to voice or data which isto be transmitted over a distance between, e.g., two components in acomputer, two computers in a network, or two phones across the countryor the world. The light travels in an optical fiber to a location whereit is detected by an optical sensor which outputs voltage that varies inaccordance with the modulation of the optical beam. In this manner,information can be rapidly transported from one location to another.

[0006] Accordingly, various components have been developed to processand manipulate optical signals. Examples of such components includemodulators, switches, filters, multiplexers, demultiplexers to name afew. Other useful optical components include lasers and opticaldetectors as well as waveguides. Many of these components can be formedon a substrate. It is therefore highly desirable to combine a variety ofsuch components into a system that is integrated onto a singlesubstrate. In such a system, optical waveguides theoretically could beused to propagate optical signals between components on the substrate.

SUMMARY OF THE INVENTION

[0007] One aspect of the present invention comprises a strip loadedwaveguide comprising a slab portion having a first refractive index n₁,a strip portion having a second refractive index n₂, and a transitionportion between the slab portion and the strip portion. The transitionportion has a refractive index n₃ that is less than the first refractiveindex n₁ and the second refractive index n₂.

[0008] Another aspect of the present invention comprises a strip loadedwaveguide comprising a slab portion and a strip portion. The stripportion is disposed with respect to the slab portion to form a guidingregion. A first portion of the guiding region is in the strip portion,and a second portion of the guiding region is in the slab portion. Theguiding region propagates light in a single spatial mode and only in atransverse electric mode.

[0009] Another aspect of the present invention comprises a strip loadedwaveguide comprising a slab portion and a strip portion. The stripportion is disposed with respect to the slab portion to form a guidingregion. A first portion of the guiding region is in the strip portion,and a second portion of the guiding region is in the slab portion. Theguiding region propagates light in a single spatial mode with across-sectional power distribution profile having two intensity maxima.A first intensity maxima is located in the slab portion, and the secondintensity maxima is located in the strip portion.

[0010] Another aspect of the present invention comprises a waveguidehaving a guiding region for guiding light through the waveguide. Theguiding region comprises a layer of polycrystalline silicon juxtaposedwith a layer of crystal silicon.

[0011] Yet another aspect of the present invention comprises anapparatus comprising a strip loaded waveguide, a transistor, and asubstrate. The strip loaded waveguide comprises a slab portion having afirst refractive index n₁, a strip portion having a second refractiveindex n₂, and a transition layer between the slab portion and the stripportion. The transistor comprises first and second portions and adielectric layer therebetween. The dielectric layer of the transistorand the transition layer of the waveguide comprise the same material.The substrate supports both the transistor and the waveguide.

[0012] Yet another aspect of the present invention comprises anapparatus comprising a strip loaded waveguide, a transistor, and asubstrate. The strip loaded waveguide comprises a slab portion having afirst refractive index n₁ and a strip portion having a second refractiveindex n₂. The transistor comprises first and second portions and adielectric layer therebetween. The second portion of the transistor andthe slab portion of the waveguide are formed of a single layer ofmaterial. The substrate supports both the transistor and the waveguide.

[0013] Still another aspect of the present invention comprises a methodof changing the index of refraction of a strip loaded waveguidecomprising a semiconductor slab and a conductive strip that areseparated by an insulating layer. The method comprises dynamicallychanging the carrier distribution in the semiconductor slab.

[0014] Still another aspect of the present invention comprises awaveguide apparatus. The waveguide apparatus comprises a slab portionhaving a first refractive index, a strip portion having a secondrefractive index, and a transition portion between the slab portion andthe strip portion. The transition portion has a third refractive indexthat is less than the first refractive index and the second refractiveindex. The waveguide apparatus additionally comprises a voltage sourceconfigured to apply a voltage between the strip portion and the slabportion such that an electric field is introduced between the stripportion and the slab portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Preferred embodiments of the present invention are describedbelow in connection with the accompanying drawings, in which:

[0016]FIG. 1 is a schematic illustration of a generic subsystemcomprising a plurality of components connected together via opticalwaveguides;

[0017]FIG. 2 is a perspective cutaway view of a strip loaded waveguidecomprising a slab having a relatively high refractive index, a stripalso having a relatively high refractive index formed on the slab, and atransition layer having a relatively low refractive index positionedbetween the slab and the strip;

[0018]FIG. 3 is a cross-sectional view of a strip loaded waveguidefurther including a map of an exemplary magnetic field distributioncorresponding to the fundamental mode supported by the strip loadedwaveguide;

[0019]FIG. 4 is a plot on axes of intensity (in arbitrary units) andposition, Y, (in arbitrary units) juxtaposed adjacent a cross-sectionalview of the strip loaded waveguide showing the optical intensity profileof the fundamental within the waveguide structure;

[0020]FIG. 5 is a cross-sectional schematic illustration of a striploaded waveguide and a transistor fabricated on the same substrate;

[0021]FIG. 6 is a cross-sectional schematic illustration of a striploaded waveguide including gate spacers;

[0022]FIG. 7 is a cross-sectional schematic illustration of a striploaded waveguide configured to be biased electronically; and

[0023]FIG. 8 is a cross-sectional schematic illustration of analternative strip loaded waveguide configured to be biasedelectronically so as to alter the index of refraction predominatelywithin the strip.

[0024]FIG. 9 is a cross-sectional schematic illustration of a striploaded waveguide comprising a polysilicon strip on a crystal siliconslab and not including a low-index transition layer therebetween.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] One preferred embodiment of the present invention comprises anintegrated optical subsystem formed on a substrate. Such subsystems maybe part of a larger optical system which may or may not be formed on asingle substrate. FIG. 1 illustrates a generic integrated opticalsubsystem 140 formed on the surface of substrate 130. The substrate 130may serve as a platform for the integrated optical subsystem 140, andthus preferably comprises a volume of material of sufficient thicknessto provide physical support for the integrated optical subsystem 140.This substrate preferably comprises a material such as silicon orsapphire.

[0026] In the embodiment illustrated in FIG. 1, a plurality ofcomponents 100 are connected by one or more integrated opticalwaveguides 110 and a splitter 120. The components 100 may compriseoptical components, electronic components, and optoelectronic orelectro-optic components. The optical and electro-optical components mayinclude waveguide devices or non-waveguide devices, i.e., light maypropagate through such components and be guided or unguided. Examples ofoptical, electro-optic, and optoelectronic components include, but arenot limited to, light sources, detectors, modulators, reflectors,polarizers, phase shifters, filters, and mode-converters.

[0027] The integrated optical waveguides 110 may be arranged in anyconfiguration to connect components 100 as desired for a particularapplication. For example, an optical signal from a one component can betransmitted to a plurality of other components through the use ofsplitter 120, as shown. The variety of configurations of waveguides andcomponents is unlimited. Waveguides can follow different paths and canbend and turn, split, cross, and can be combined. Different components,electrical, optical, electro-optic, and optoelectronic can be includedon the substrate, and in various embodiments, can be optically coupledto the waveguides and to each other. In addition, electrical connectionscan be made to the components and to the waveguides as is discussed morefully below. The arrangement of waveguides and components is not to beconsidered limited but may include any variety of combinations andjuxtapositions.

[0028] In some embodiments, the substrate 130 will support a pluralityof material layers which together create layers of integrated opticalsubsystems stacked atop each other. Each of these layered integratedoptical subsystems may include waveguides and/or components, electrical,optical, electro-optic, and optoelectronic, formed within a given layer.Such multi-layered stacking will add to the variety of integratedoptical designs that are possible. Light can be directed between thevarious layers using waveguides situated therebetween, gratings such asfor example waveguide gratings, and Bragg diffaction elements such asdistributed Bragg gratings. Multilayer optical films such as thin filmfilters can be incorporated to introduce the desired phase delay and maybe used to enable various functionalities, such as for example opticalfiltering. The structures and methods involved in coupling light fromone layer to another, however, are not limited to those recited herein.

[0029] In general, optical waveguides comprise a core region comprisingmaterial that is at least partially transparent. This core region issurrounded by a cladding region that confines light within the coreregion. Some optical energy, often referred to as the evanescent energyor the evanescent field, however, may exist outside the core region andwithin the cladding region.

[0030] In certain waveguides, the core region comprises a first materialhaving a first refractive index, and the cladding region comprises asecond material having a second refractive index, the refractive indexof the core region being greater than the refractive index of thecladding region. A core/cladding interface is located at the boundarybetween the core region and the cladding region. In such embodiments,when light in the core region is incident upon this core/claddinginterface at an angle greater than the critical angle, the light isreflected back into the core region. This effect is referred to as totalinternal reflection. In this manner, optical signals can be confinedwithin the core region due to total internal reflection at thecore/cladding interface.

[0031] Waveguides can be fabricated in a wide variety of geometries andconfigurations. An optical fiber is a specific type of waveguide thatfits the description above. An optical fiber generally comprises acircularly cylindrical core surrounded by an circularly cylindrical orannular cladding layer. The core has a relatively high refractive indexand the cladding has a relatively low refractive index. The core andcladding may comprise, e.g., silica or silica based materials, and aretypically flexible, with core diameters of approximately 10 μm forsingle-mode fiber. As discussed above, optical fibers are often used totransmit optical signals across large distances, ranging for examplefrom centimeters to thousands of kilometers.

[0032] Optical fibers should be distinguished from integrated opticalwaveguides, which are generally associated with a substrate. Theintegrated optical waveguide may for example be situated on thesubstrate, in a substrate, or partially on and partially in thesubstrate. The integrated optical waveguide may be part of the substrateitself but preferably comprises of one or more layers of materialpositioned on a surface of the substrate. Examples of integrated opticalwaveguides include channel waveguides, rib or ridge waveguides, slabwaveguides, and strip loaded waveguides, all of which are well-known inthe art. In contrast to optical fibers, integrated optical waveguidesare less likely to have a circularly symmetric cross-section although intheory they can be circularly cylindrical. Additionally, integratedoptical waveguides are generally used to transmit optical signalsbetween locations on the substrate, and thus preferably have lengthsranging from microns to centimeters.

[0033] In accordance with conventional usage in the art, opticalcomponents that are integrated onto a substrate with integrated opticalwaveguides, are collectively referred to herein as integrated optics.Such optical component may for example, process, manipulate, filter orotherwise alter or control optical signals propagating within thewaveguides. As discussed above, these components themselves may bewaveguides that guide light.

[0034] One of the simplest integrated optical waveguide configurationsis the conventional slab waveguide. The slab waveguide comprises a thin,planar slab surrounded by cladding regions. The cladding regions maytake the form of first and second (for example, upper and lower)cladding layers on either side of the slab. The two cladding layers neednot comprise the same material. In this simplified example, the slab maybe planar with substantially parallel planar boundaries at theinterfaces with the first and second cladding layers. Generally, theslab has a higher refractive index than either of the cladding layers.Light can therefore be confined in one dimension (e.g., vertically)within the slab. In this configuration of the slab waveguide, opticalenergy is not confined laterally to any portion of the slab, but extendsthroughout the slab due to total internal reflection at the planarboundaries between the slab and the surrounding upper and lower claddinglayers.

[0035] A strip loaded waveguide is formed by positioning a strip on theslab of a slab waveguide. The slab and the strip located thereon may besurrounded on opposite sides by the first and second (e.g., upper andlower cladding layers). Preferably, the strip has a refractive indexthat is greater than that of either cladding layer, however, the indexof the strip is preferably approximately equal to that of the slab. Thepresence of the strip positioned on the slab induces an increase ineffective index of the slab in the region beneath the strip and inproximity thereto.

[0036] Accordingly, the region within the slab that is beneath the stripand in proximity thereto has a higher effective refractive index thanother portions of the slab. Thus, unlike the slab waveguide whereinoptical energy propagates throughout the planar slab, the strip loadedwaveguide substantially confines optical energy to the region of theplanar slab layer under the high-index strip. In a strip loadedwaveguide, therefore, an optical signal can be propagated along a pathin the slab defined by the region over which the high-index strip isplaced on the slab. Thus, slab waveguides defining any number andvariations of optical pathways, can be created by depositing one or morestrips onto the slab having the shape and orientation of the desiredoptical pathways.

[0037]FIG. 2 is a schematic cutaway illustration of a preferredembodiment of a strip loaded waveguide 200. The strip loaded waveguide200 comprises a slab 205 having a first refractive index n₁ and a strip210 having a second refractive index n₂. In addition, the strip loadedwaveguide 200 has a transition layer 215 having a third refractive indexn₃. The transition layer 215 is positioned between the slab 205 and thestrip 210, such that the slab 205 and the strip 210 do not directlycontact each other. The refractive index of the transition layer n₃ isless than the refractive index of the slab n₁ and the refractive indexof the strip n₂.

[0038] In certain embodiments of the invention, semiconductor materialsused in conventional processes for fabrication of semiconductormicroelectronics are employed to create strip loaded waveguides. Thesematerials include, but are not limited to, crystalline silicon,polysilicon and silicon dioxide (SiO₂). In particular, in one preferredembodiment, the slab 210 comprises single crystal silicon, thetransition layer 215 comprises silicon dioxide and the strip 210comprises polysilicon, although in other embodiments, the strip 210 maycomprise crystal silicon. The crystal silicon slab 215 and thepolysilicon strip 210 may be doped, for example, in cases where the slab215 or the strip 210 are to be electronically conductive. Inapplications where the slab 215 or the strip 210 need not beelectronically conductive, the slab 215 and the strip 210 are preferablyundoped to minimize absorption losses.

[0039] As is well known, single crystal silicon is used to fabricatesemiconductor microelectronics and integrated circuits (ICs), such asmicroprocessors, memory chips, and other digital as well as analog ICs,and thus single crystal silicon is well characterized and its propertiesare largely well understood. The term single crystal silicon is usedherein consistently with its conventional meaning. Single crystalsilicon corresponds to crystalline silicon. Single crystal silicon,although crystalline, may include defects such that it is not truly aperfect crystal, however, silicon having the properties conventionallyassociated with single crystal silicon will be referred to herein assingle crystal silicon despite the presence of such defects. The singlecrystal silicon may be doped either p or n as is conventional. Suchdoping may be accomplished, for example, by ion implantation.

[0040] Single crystal silicon should be distinguished from polysiliconor “poly”. Polysilicon is also used to fabricate semiconductormicroelectronics and integrated circuits. The term polysilicon or “poly”is used herein consistently with its conventional meaning. Polysiliconcorresponds to polycrystalline silicon, silicon having a plurality ofseparate crystalline domains. Polysilicon can readily be deposited forexample by CVD or sputtering techniques, but formation of polysliconlayers and structures is not to be limited to these methods alone.Polysilicon can also be doped p or n and can thereby be madesubstantially conductive. In general, however, bulk polysilicon exhibitsmore absorption losses in the near infrared portion of the spectrum thana similar bulk single crystal silicon, provided that the doping,temperature, and other parameters are similar.

[0041] As illustrated in FIG. 2, the strip loaded waveguide 200 ispreferably located on a supporting structure 220 or substrate. Thesupporting structure 220 serves to support the strip loaded waveguide200 and preferably comprises a material such as a silicon or sapphiresubstrate 222. Additionally, the supporting structure 220 may alsoinclude a cladding layer 224, which aids in confining optical energywithin the slab portion 205. Accordingly, this layer 224 preferably hasa refractive index that is low in comparison to the refractive index ofthe slab 205.

[0042] In one preferred embodiment, the supporting structure 220comprises a silicon substrate 222 having a cladding layer 224 of silicondioxide formed thereon. The silicon dioxide layer 224 on the siliconsubstrate 222 with an index of approximately 1.5 serves as a lowercladding layer for the slab 205 having an index of approximately 3.5.This silicon substrate 222 may comprise doped silicon and may be acommercially available silicon wafer used for fabricating semiconductorintegrated circuits. In other embodiments, the cladding layer 224 maycomprise silicon nitride. The index of refraction of silicon nitride isapproximately 1.9.

[0043] In alternative embodiments, wherein the supporting structure 220comprises a material other than silicon, the cladding layer 224 ofsilicon dioxide may not be present. For example, the slab 205 may restdirectly on a sapphire substrate 222. Processes for growing crystalsilicon on sapphire have been developed. In general, in these cases, thesupporting structure 220 preferably has an index of refraction lowerthan that of the slab 205. In other embodiments, an additional claddinglayer 224 may be formed on the these non-silicon substrates.

[0044] The slab 205 is therefore disposed either on the substrate 222 oron a layer 224 (preferably the cladding) formed over the substrate. Thiscladding layer 224 itself may be formed directly on the substrate 222 ormay be on one or more layers formed on the substrate 222. The slabportion 205 may span the substrate 222 or extend over only a portion ofthe substrate 222. As discussed above, the slab 205 preferably comprisessingle crystal silicon and has an index of refraction n₁ on average ofabout 3.5 and has a thickness t₁ preferably between about${\frac{\lambda}{6n}\quad {and}\quad \frac{\lambda}{4n}},$

[0045] and more preferably about $\frac{\lambda}{4n},$

[0046] where n is the index of refraction. This thickness, t₁,determines in part the optical mode or modes supported by the striploaded waveguide and depends partially on the geometry of the structure.In alternative embodiments, the slab 205 may comprise materials otherthan single crystal silicon and may be doped or undoped and thus mayhave different refractive indices. The slab 205, however, preferablycomprises crystal silicon. Localized doping, such as used to create thesource, drain, and channel regions in a transistor, may affect theoptical properties of the slab 205. The index of refraction in localizedregions of the slab can vary slightly due to doping by ion implantation.

[0047] In general, the strip 210 is disposed above and in a spaced-apartconfiguration with respect to the slab 205. The strip 210 may comprisedoped polycrystalline silicon having an index of refraction n₂ ofapproximately 3.5. In alternative embodiments, the strip 210 maycomprise doped single crystal silicon having an index of refraction n₂on average about 3.5. As discussed above, however, the strip may also beundoped and may comprise materials other than polysilicon or crystalsilicon although these materials are preferred. An example of one suchalternative material that may used to form the strip 210 is siliconnitride (Si₃N₄), which has an index of refraction n₃ of approximately1.9.

[0048] The dimensions of the strip 210 may vary and depend in part onthe overall composition and geometry of the waveguide. As with the slab205, however, the size of the strip 210 determines in part the number ofmodes to be supported by the waveguide and the wavelength of thesemodes.

[0049] The transition layer 215 is positioned between the slab 205 andthe strip 210. This transition layer 215 may span the slab 205 asillustrated in FIG. 2 or extend over only a portion of the substrate205. Preferably, the refractive index of the transition layer 215 isless than the refractive index of the polysilicon strip 210 and thecrystalline silicon slab 205. In one preferred embodiment, thetransition layer 215 comprises silicon dioxide having an index ofrefraction n₃ of approximately 1.5.

[0050] In various embodiments, the transition layer 215 may includeoptically active (i.e., gain inducing) material, such as erbium.Waveguide structures that include an optically active gain inducingmaterial in the transition layer 215 can produce gain and amplify orregenerate the strength of the optical signal propagating through thewaveguide. Specialized components can be formed using these amplifyingstructures.

[0051] In certain embodiments, the thickness t₃ of the transition layer215 is equal to the thickness of the gate oxide layer of transistors(not shown) positioned on the same substrate as the strip loadedwaveguide 200 and fabricated in the same process as the strip loadedwaveguide 200. The width of the transition layer 215 may besubstantially equal to the width w₂ of the strip 210, although in otherembodiments, such as illustrated in FIG. 2, the width of the transitionlayer 215 is greater than the width w₂ of the strip 210.

[0052] In the waveguide structure illustrated in FIG. 2, the striploaded waveguide 200 is covered by one or more coatings 230, althoughthese coatings are optional. Two coatings are shown in FIG. 2, one withan index of refraction n₄ and another thereon with an index ofrefraction n₅. More or less coatings may be used and in otherconfigurations the coatings 230 can be excluded and replaced insteadwith air or vacuum. The optional nature of these coatings 230 isemphasized by depicting the coating in phantom in FIG. 2. These coatings230, however, are useful for protecting the strip loaded waveguide 200from damage or interference which may occur due to contact with otherobjects. Accordingly, the coatings 230 preferably completely covers thestrip loaded waveguide 200, although in other case, the coating mayextend only over portions of the strip 210 or slab 205.

[0053] The coatings 230 may also serve as a cladding layer, providingconfinement of optical energy within the slab 205 and the strip 210.Accordingly, the coatings 230 preferably have indices of refraction n₄,n₅ less than that of the slab 205 and the strip 210. The coatings 230may have an index or refraction equal to that of the low-indextransition layer 215 and may comprise the same material as the low-indextransition layer 215. Alternatively, the coatings 230 may have adifferent indices of refraction than the transition layer 215 and maycomprise different material. In multilayered integrated opticalstructures, the coatings 230 may serve as a substrate for second striploaded waveguide in a layer disposed above a first strip loadedwaveguide.

[0054] Accordingly, the coatings 230 preferably comprises a solid,possibly electrically insulating material, having a refractive indexless than that of the slab 205 and the strip 210. The coatings 230 may,for instance, comprise glass or silicon dioxide. Other materials and,more specifically, other dielectrics may also be employed. Polymericmaterial, such as for example polyimide may be used in certainapplications.

[0055] Confinement of light within the slab 205 is provided because theslab 205 has a higher refractive index than the layers above and below.In one preferred embodiment, for example, light is confined within thesilicon slab 205 because the silicon slab 205 has a higher refractiveindex than the glass coatings 230 covering it. In addition, the siliconslab 205 has a higher index than the silicon dioxide cladding layer 224immediately below it.

[0056] The light within the slab 205 is confined to portions beneath thestrip 210 because of the presence of the strip 210, despite the factthat the strip 210 is separated from the slab 205. The interveningtransition layer 215 does not prevent the strip 210 from determining theshape and location of the optical mode(s) supported in the slab 205. Thepresence of the strip 210 positioned proximally to the slab portion 205induces an increase in effective index of the slab portion 205 in theregion directly under the strip 210 and in proximity thereto. Thisincrease in effective index defines a relatively high effective indexguiding region 225 wherein light in one or more supported optical modesis guided along the strip loaded waveguide 200. The strip loadedwaveguide 200 guides supported modes in the guiding region 225 despitethe presence of the transition layer 215 between the slab 205 and strip210. In particular, the transition layer 215 does not prevent the strip210 from altering the effective index within the slab 205 and moreparticularly, from raising the effective index within the slab 205.Preferably, the transition layer 215 has a thickness sufficiently smallsuch that the strip 210 can increase the effective index of the slab 205in regions immediately beneath and in the proximity thereto. Thetransition layer 215 is sufficiently thin and the strip 210 and the slab205 are sufficiently close, although physically separated by theintervening transition layer, that the strip 210 can affect thepropagation of light within the slab 205. The transition layer 215 alsopreferably has an index of refraction that is low in comparison withthat of the strip 210 and the slab 205.

[0057] The guiding region 225 corresponds to a boundary where a specificportion of the optical energy within the mode, preferably thefundamental mode, is substantially contained and thus characterizes theshape and spatial distribution of optical energy in this mode.Accordingly, the guiding region 225 corresponds to the shape andlocation of the optical mode or modes in this strip loaded waveguide200. In the guiding region 225, the electric field and the opticalintensity are oscillatory, where as beyond the guiding region 225, theevanescent field exponentially decays.

[0058] Propagation of an optical signal in the strip loaded waveguide200 illustrated in FIG. 2 is further characterized by the spatialdistribution of the field strength across the cross-section of the striploaded waveguide 200. FIG. 3 illustrates the magnetic field distributionacross a cross-section of the waveguide 200 parallel to the x-y plane.This distribution is the result of modeling using finite difference timedomain iterations to calculate the horizontal component of the magneticfield in the mode supported by the structure, i.e., the fundamentalmode. The electric field is vertically polarized in this example. Thecase where the transition layer has the same refractive index as theregion surrounding the slab was modeled. As shown, the field strengthwithin the fundamental mode is distributed within the slab 205 despitethe presence of the transition layer 215 and the separation between thestrip 210 and the slab 205. The field, however, is localized within thestrip 210 and in the slab 205 within a region proximal to the strip.This field strength distribution is consistent with the guiding region225 shown in FIG. 2.

[0059] A schematic diagram of the intensity through the thickness of thewaveguide structure is presented in FIG. 4. This plot shows the opticalenergy substantially confined within the strip 210 and the region of theslab 205 below and adjacent to the strip 210.

[0060] The intensity profile shown in FIG. 4 is characterized by thepresence of a localized intensity minima 235 in the lowest-order guidedmode. The localized intensity minima 235 occurs in the proximity of thetransition layer 215 between the strip 210 and the slab 205.Accordingly, this localized minima 235 is likely caused by the presenceof the transition layer 215 and the separation of the slab 205 from thestrip 210. Nevertheless, the presence of the transition layer 215 doesnot substantially disrupt the mode. Optical energy can still bepropagated along a guiding region 225 partially within the strip 210 andthe slab 205. Accordingly, the propagation of light can be controlledand beams can be directed along pathways defined by these strip loadedoptical waveguides 200. Integrated optical systems can therefore beconstructed wherein light is guided to and from components and therebymanipulated and processed as desired.

[0061] Such integrated optical systems can be fabricated usingwaveguides similar to those disclosed herein. It will be appreciatedthat, although the strip loaded waveguide 200 illustrated in FIG. 2 hasa substantially straight configuration, it will be understood that inalternative embodiments, the strip loaded waveguide can have anunlimited variety of alternative configurations and orientations,including but not limited to bends and turns, and intersections withother strip loaded waveguides. See, e.g., FIG. 1. Additionally, althoughthe strip loaded waveguide 200 illustrated in FIG. 2 has a rectangularcross-section (parallel to in the x-y plane), other cross-sectionalgeometries can be used, such as a trapezoidal, elliptical, orrectangular. Also, although not shown in the drawings, the corners andedges may be rounded or otherwise irregularly shaped.

[0062] As indicated above, an optical signal confined within the striploaded waveguide 200 can be coupled from or is coupled to other opticalcomponents, such as for example modulators, switches, and detectors, atwaveguide input ports and the waveguide output ports. These opticalcomponents may be waveguide structures having the features describedabove. Such configurations allow for further processing or transmissionof the optical signal.

[0063] Furthermore, multiple strip loaded waveguides can be positionedatop each other on the substrate, thereby forming a layered integratedoptic structure. Accordingly, a plurality of strip loaded waveguides canbe combined into a system comprising waveguide networks, thus allowingoptical signals to be coupled between components. The specifications ofsuch alternate configurations may be determined by the particularapplication in which the strip loaded waveguide structure is to be used.

[0064] Advantageously, the specific material systems that can be used toimplement these strip loaded waveguides have numerous desirablefeatures. Single crystal silicon and polycrystalline silicon aresubstantially transparent at wavelengths in the near infrared spectrum(i.e., between approximately 1.3 μm and 1.6 μm) and thus provide anefficient medium for the propagation of near infrared light. Thecombination of silicon (crystalline or polysilicon) and silicon dioxidealso possesses a high refractive index contrast, i.e., the differencebetween the refractive index of the materials is relatively large. Inparticular, the index of refraction of crystalline silicon andpolysilicon is about 3.5 depending on a variety of parameters. Incontrast, silicon dioxide has an index of refraction of about 1.5. Thisdisparity in refractive index between silicon and silicon dioxide isapproximately 2.0, and is large in comparison for example with thedisparity in refractive index between the silica core and silicacladding that make up conventional optical fiber, both of which areabout 1.5. The difference between the refractive indices of the core andcladding in silica based fiber is approximately 0.003. Thiscore/cladding index difference in the strip loaded waveguides describedabove that comprise silicon and silicon dioxide are approximately threeorders of magnitude higher than that of silica optical fiber. In otherembodiments, the core/cladding index difference is preferably at leastabout 1.0. High index contrast is advantageous because it providesincreased optical confinement of the light within the waveguide.Accordingly, high index contrast allows waveguides having substantiallysmaller dimensions to be employed. Additionally, sharper bends andsmaller bend radii can be incorporated into the waveguides with outexcessive losses.

[0065] In addition, certain of the embodiments of the strip loadedwaveguide can be fabricated using conventional integrated circuitfabrication processes. For instance, the supporting structure 220 maycomprise a commercially available silicon wafer with silicon dioxideformed thereon. Conventional “Silicon-on Oxide” (SOI) processes can beemployed to form the silicon slab 205 on a silicon wafer or on asapphire substrate. Fabrication techniques for forming the a crystalsilicon layer on an insulator include, but are not limited to, bondingthe crystal silicon on oxide, SIMOX (i.e., use of ion implantation toform oxide in a region of single crystal silicon), or growing silicon onsapphire. Oxide formation on the silicon slab can be achieved withconventional techniques for growing gate oxides on a silicon activelayers in field effect transistors (FETs). Still other processesutilized in fabricating FETs can also be applied. In the same fashionthat a polysilicon gate is formed on the gate oxide in field effecttransistors, likewise, a polysilicon strip can be formed over the oxidetransition region in the strip loaded waveguide. This polysilicon stripcan be patterned using well-known techniques such as photolithographyand etching. Damascene processes are also considered possible.Accordingly, conventional processes such as those employed in thefabrication of Complementary Metal Oxide Semiconductor (CMOS)transistors can be used to create the waveguide. In other embodiments,crystalline silicon strips can be formed on the transition oxide regionusing conventional techniques such as SOI processing.

[0066] Another processing advantage is that in the fabrication ofpolysilicon or silicon strips 210, the transition layer 215 thatseparates the slab 205 from the strip 210 may in some cases act as anetch stop. For example, in applications where the strip 210 and the slab205 are etched from the same material, the etch can be configured tostop on the thin transition layer 215 therebetween. This fabricationconfiguration allows the geometry of the waveguide to be accuratelycontrolled without having to dynamically control the etch depth.

[0067] Another strategy for fabricating the strip loaded waveguide is toobtain a commercially available SOI wafer which comprises a firstsilicon substrate having a first silicon dioxide layer thereon with asecond layer of silicon on the first silicon dioxide layer. Theaggregate structure therefore corresponds to Si/SiO₂/Si. The firstsilicon dioxide layer is also referred to as the buried oxide or BOX. Asecond silicon dioxide layer can be formed on the SOI wafer andpolysilicon or silicon strips 210 can be formed on this structure tocreate strip loaded waveguides 200 with the second silicon layercorresponding to the slab 205 and the second silicon dioxide layerformed thereon corresponding to the transition layer 215. The thicknessof this second silicon dioxide transition layer can be controlled asneeded. The polysilicon or silicon strips can be patterned for exampleusing photolithography and etching. Damascene processes are alsoenvisioned as possible.

[0068] In the case where the substrate does not comprise silicon (with alayer of silicon dioxide on the surface), a slab comprising crystalsilicon can still be fabricated. For example, crystalline silicon can begrown on sapphire. The sapphire will serve as the lower cladding for theslab. Silicon nitride formed for example on silicon can also be acladding for the slab. The formation of the transition layer and thestrip on the silicon slab can be performed in a manner as describedabove.

[0069] Other conventional processes for forming layers and patterningmay also be used and are not limited to those specifically recitedherein. Employing conventional processes well known in the art isadvantageous because the performance of these processes is wellestablished. SOI and CMOS fabrication processes, for example, are welldeveloped and well tested, and are capable of reliably producing highquality products. The high precision and small feature size possiblewith these processes should theoretically apply to fabrication ofstrip-loaded waveguides as the material systems are similar.Accordingly, extremely small sized waveguide structures and componentsshould be realizable, thereby enabling a large number of such waveguidesand other components to be integrated on a single die. Althoughconventional processes can be employed to form the strip loadedwaveguides described herein, and moreover, one of the distinctadvantages is that conventional semiconductor fabrication processes canreadily be used, the fabrication processes should not be limited to onlythose currently known in art. Other processes yet to be discovered ordeveloped are also considered as possibly being useful in the formationof these structures.

[0070] Another advantage of these designs is that in various embodimentselectronics, such as transistors, can be fabricated on the samesubstrate as the strip loaded waveguides. Additionally, integration ofwaveguides and electronics on the same substrate is particularlyadvantageous because many systems require the functionality offered byboth electronic, optical, electro-optical, and optoelectroniccomponents. For example, with the waveguide structures describe herein,modulators, switches, and detectors, can be optically connected togetherin a network of waveguides and electrically connected to control anddata processing circuitry all on the same die. The integration of thesedifferent components on a single die is particularly advantageous infacilitating minimization of the size of devices, such as opticaltelecommunications devices.

[0071] The integration of integrated optical components and withelectronics on a single die is illustrated in FIG. 5, which depicts across-sectional view of a strip loaded waveguide 300 disposed on asubstrate 320 that also supports a field effect transistor 350. Asdiscussed above, this substrate 320 may comprise a silicon wafer havinga silicon dioxide surface layer, or a sapphire substrate. A siliconlayer 305 is formed on the silicon substrate 320, and more particularlyon the silicon dioxide surface layer of the substrate. This siliconlayer 305 corresponds both to the slab of the strip loaded waveguide 300and the active silicon of the transistor 350. Accordingly, both the slaband the active silicon of the transistor 350 where the channel is formedpreferably comprise the same material and substantially the samethickness although the thicknesses may vary in some embodiments. Bothmay comprise a doped semiconductor. The localized doping concentrationsmay vary slightly as the transistor will include source, drain andchannel regions with different doping than that of the remainder of thesemiconductor layer.

[0072] A thin oxide layer 315 is formed on the silicon layer 305. Thisthin oxide layer 315 corresponds to the transition layer of the striploaded waveguide 300 and the gate oxide of the field effect transistor350. Accordingly, the transition layer of the strip loaded waveguide 300and the gate oxide of the FET 350 preferably comprise the same materialand preferably have substantially the same thickness although thethicknesses may vary in some embodiments.

[0073] A patterned polysilicon layer 310 can be formed on the thin oxidelayer 315. This patterned polysilicon layer 310 includes both the stripon the strip loaded waveguide 300 and the gate on the field effecttransistor 350. In other embodiments, the gate of the transistorcomprises single crystal silicon. Likewise, the strip of the striploaded waveguide 300 and the gate of the transistor 350 preferablycomprise the same material and have substantially the same thicknessalthough the thicknesses may vary in some embodiments. The strip,however, may be an elongated structure to facilitate the propagation oflight along a pathway from one location to another on the integratedoptical chip. Likewise, this polysilicon or crystal silicon strip mayturn and bend, and split or be combined with other strips. In contrast,the transistor gate is preferably not elongated and may be more squarethan the strip (as seen from the top, i.e., in a plane parallel to thex-z plane shown in the drawings). The shapes of the strips are notrestricted to square or even rectangle (as seen from a top) as bends andturns and splitting and combining as well as intersections may beincluded among the many functionalities of the waveguides. Additionally,transistors often use salicides to enhance conductivity at ohmiccontacts. In contrast, unless electrical connections are to be formed onthe waveguides, the waveguide structure preferably does not includesalicides so as to reduce absorption losses.

[0074] Advantageously, in such embodiments the strip loaded waveguide300 and the transistor 350 can be fabricated using the same fabricationprocesses. For example, the same substrate may be employed. The slab 305of the waveguide 300 and the active silicon of the transistor 350 can beformed by the same silicon growth, deposition or other formationprocess. Similarly, the transition layer 315 and the gate oxide can begrown or formed in the same processing step. The strip 310 and gate canbe created both by patterning polysilicon (or crystal silicon) at thesame stage of the process. Accordingly, substantially the samefabrication processes can be used to produce both the transistors andthe waveguides. In fact, these structures can be realized substantiallysimultaneously.

[0075] In the fabrication of certain semiconductor electronics, it maybe desired to provide spacers such as, for example, silicon nitridespacers. In particular, gate spacers positioned adjacent to the gate ofthe field-effect transistor (“FET”) prevent unwanted doping below thegate. This unwanted doping may result from ion implantation employed todope source and drain regions adjacent the gate. In embodiments whereinstrip loaded waveguides and electronic components are formed on the samesubstrate using the same fabrication process, it will often be desirableto fabricate gate spacers on both the strip loaded waveguides as well asthe electronic components although such spacers can be included evenwhen the transistors are not present on the chip.

[0076]FIG. 6 illustrates one preferred embodiment of a strip loadedwaveguide 500 having spacers 545. The strip loaded waveguide 500comprises a slab 505, a strip 510, and a transition layer 515therebetween. The strip loaded waveguide 500 is disposed on substrate520 which may include a dielectric layer corresponding to the lowercladding of the strip loaded waveguide 500. Spacers (e.g., gate spacers)545 are fabricated adjacent to the strip 510. The spacers 545 maycomprise a nitride or an oxide, although other preferably nonconductivematerials can be used in other embodiments. In addition to preventingion doping in regions proximal to the gate layer in transistors, incertain circumstance, the spacers 545 may prevent doping in the regionbeneath strip. The spacers can also be used to alter the effective indexin the slab and to thereby adjust the confinement within the guidingregion and/or to prevent salicide from forming near the waveguide.

[0077]FIG. 6 also shows liners 550 between the spacers 545 and the strip510. These liners 550 may comprise, for example silicon dioxide, and maybe used as passivation for the strip or gate 510. The liners may alsoact as etch-stop layers. In alternative embodiments, the liners 550 maynot be present, and the spacers 545 may be in direct contact with thestrip 510.

[0078] In various embodiments, the index of refraction of the striploaded waveguide can be actively controlled with an applied field. FIG.7 illustrates such a configuration wherein a voltage can be appliedacross a strip loaded waveguide 400. The strip loaded waveguide 400includes a slab 405, preferably comprising crystalline silicon, and astrip 410, preferably comprising polysilicon or crystalline silicondisposed on a substrate 420. The silicon slab 405 and the poly orsilicon strip 410 are preferably doped so as to be conductive. Asdescribed above, a thin transition layer 415, comprising for examplegate oxide such as silicon dioxide, separates the strip 410 and the slab405. A dielectric coating 430, which may be formed from multiple layers,covers the strip 410 and slab 405 and provides electrical insulation.Conductive plugs 445 within the dielectric provide a substantiallyconductive pathways to the poly or silicon strip 410 and the siliconslab 405. Salicide or metalization 460, and/or ohmic contacts 440, canbe formed on or in the polysilicon or silicon strip 410 or the siliconslab 405 to electrically couple the plugs 445 to these portions of thestrip loaded waveguide 400. A voltage source 435 is electricallyconnected to the plugs 445.

[0079] Application of a voltage between the polysilicon or silicon strip410 and the silicon slab 405 causes carriers 450 to accumulate withinthe guiding region 425 of the strip loaded waveguide 400. For example,depending on the applied voltage, its polarity, and the doping of thestrip 410 and the slab 405, electrons or holes may accumulated or bedepleted within the strip 410 or the slab 405 in regions adjacent to thethin transition layer 415 comprising gate oxide. The structure acts likea capacitor, charging with application of a voltage. The voltage createsan electric field across the thin transition layer 415 with carriers 450accumulating (or depleting) adjacent to this transition layer 415.Preferably, the transition layer 415 is sufficiently thick such that thecarriers do not traverse this barrier layer by tunneling or throughdefects, such as pinhole defects. Conversely, the thickness of thisdielectric layer 415 is preferably not so large as to require a largevoltage to be applied to the device to generate enough carriers to varythe index of the strip loaded waveguide 400. The thickness of this layerwill also be affected by similar considerations in transistors formed onthe same layer as the strip loaded waveguide 400. For example, in fieldeffect transistors, the gate oxide is preferably sufficiently thick soas to prevent tunneling of carriers from the channel region into thegate but is sufficiently thin such that the voltage required to activatethe transistor is not too large.

[0080] The magnitude of the applied voltage and the resultant electricfield across the transition layer 415 controls the carrier density ofthe strip loaded waveguide 400. Preferably, the carrier density at leastwithin the guiding region 425 is altered by the application of thevoltage. This carrier accumulation or depletion may be concentratedpredominately in the strip 410 or the portion of the slab 405 beneaththe strip 410. The refractive index of semiconductor material alterswith variation in carrier concentration. The accumulation of carrierslowers the index of refraction while depletion of carriers raises theindex. The refractive index of the strip 410 and portions of the slab405 can therefore be altered by controlling the carrier density inregions therein. For instance, by accumulating or depleting carriers inthe proximity of the transition layer 415, the effective index of thestrip 410 and the slab 405 can be altered as desired. In addition toaffecting the refractive index, accumulation of carriers also increasesabsorption. Application of a field can therefore also vary theabsorption coefficient associated with the waveguide.

[0081] Accordingly, the optical properties of the waveguide 400 may becontrollably altered with application of an electric bias. The index ofrefraction can be varied to alter the effective optical path distancewithin the guide and adjust or tune the guide for different wavelengths,introduce or reduce phase delay, and increase or decrease opticalconfinement within the guide, or to otherwise affect the lightpropagating within the guide as desired. Since the absorption can alsobe controlled, the intensity of the light can be altered. Electronicbiasing therefore can be employed to the modulate the signal or tocreate other optical or electro-optical components which can be operatedby actively changing the index of the refraction and/or the absorptionof the waveguide or portions of it. Electronic biasing can also be usedto adjust or tune waveguide structures to account for, e.g.,manufacturing tolerances, or to configure the structure for differentapplications.

[0082] Gate spacers (not shown in FIG. 7) may further be included asdiscussed above and may minimize fringing of the electric field in thecase where a dielectric coating 230 does not cover the strip loadedwaveguide.

[0083] In an alternative configuration illustrated in FIG. 8, anadditional poly or silicon layer 470 can be formed over the strip 410with a dielectric region 475 separating this additional poly or siliconlayer and the strip. Electrical connection may be made to thisadditional poly or silicon layer 470 and to the strip 410 viametalization 460 on the additional poly or silicon layer and conductingplugs 445 though the dielectric 430 to the metalization. A conductivepathway is also provided to the strip 410 by way of metal plugs 445 andan ohmic contact 440 in the strip. Application of a voltage between theadditional poly or silicon layer 470 and the strip 410 will causecarriers 450 to accumulate or be depleted in the strip 410. Thisarrangement enables the carrier density of the strip 410 to be alteredindependent of the carrier density within the slab 405. Accordingly, theindex of refraction and/or absorption can be changed predominantlywithin the strip 410, while these properties in the slab 405 arepreferably unaltered. Other configurations suited to the particularapplication are considered possible. For example, electricallyconnection can also be made with the slab 405 and a voltage can beapplied between the strip 410 and the slab to alter the carrierdistribution below the strip and affect the index of refraction in theslab.

[0084] In each of these designs, regardless of whether the waveguide isconfigured for application of an electronic field, the properties of thesemiconductor portions can be adjusted based on how the material isdoped with impurities, if any.

[0085] Also, as discussed above with reference to FIG. 2, the dimensionsof the strip 210 and the slab 205 may vary depending on the applicationof the waveguide. For example, in an application wherein the waveguidemust be configured to propagate only a single optical mode, thedimensions of the strip 210 (and possibly the slab 205) may be adjustedaccordingly. The dimensions of the strip portion 210 and the slabportion 205 may also depend on the wavelength of the optical signalconfined in the waveguide.

[0086] In certain embodiments, the dimensions of the strip loadedwaveguide 210 can be selected such that only a single mode and singlepolarization can be propagated in the guiding region 225. These specialstrip loaded waveguides are single mode waveguides that in addition onlysupport one polarization. In one example, for instance, the dimensionsof the waveguide can be designed so as to support only thetransverse-electric (“TE”) fundamental mode. The TE mode corresponds tolight having a polarization parallel to the interface between the slab205 and transition layer 215 or the strip 210 and the transition layer215 (that is, with the electric field is parallel to the x-z plane asdefined in FIG. 2). For light having a wavelength of 1.55 μm, single TEmode operation can be obtained by configuring the thickness of the slabportion 205 to be approximately 110 nm, the thickness of the stripportion 210 to be approximately 95 nm, and the thickness of thetransition portion 215 to be approximately 40 nm. The strip 210 has awidth of about 0.5 micrometers. Finite difference time domain iterationsand eigenmode solvers can be used to determine appropriate dimensionsfor other such strip loaded waveguides that supports a single TE mode.In this particular case, the slab portion 205 and the strip portion 210both comprise single crystal silicon, and the transition portion 215comprises silicon dioxide. However, specific embodiments with differentmaterials and different dimensions can be obtained that support only asingle polarization mode. Such a configuration may be particularlyadvantageous in certain polarization-dependent applications where onlyone polarization is required. Such a waveguide, for example, can act asa linear polarizer. These waveguides that support a single polarizationof the fundamental mode may also be employed to minimize crosstalk.

[0087] In alternative embodiments, as illustrated in FIG. 9, the striploaded waveguide 600 may comprise a strip 610 formed directly on a slab605 that is supported by substrate 620. In such embodiments, nolow-index transition layer is positioned between the strip 610 and theslab 605. The presence of the strip 610 positioned adjacent to the slab605 induces an increase in effective index of the slab portion 605 inthe region directly under the strip 610 and in proximity thereto. Thisincrease in effective index defines a relatively high effective indexguiding region 625 wherein light in one or more supported optical modesis guided along the strip loaded waveguide 600. This strip 610 comprisespolysilicon and the slab 605 comprises crystal silicon. The crystalsilicon slab 605 may be formed on a oxide or nitride layer on a siliconsubstrate. Other insulator layers may be employed as the lower claddinglayer and as the substrate. For example, sapphire may be used as asubstrate with crystal silicon formed thereon. One or more layers oflower index material such as glass or oxide may be formed over the strip610 and the slab 605.

[0088] As described above, silicon is substantially opticallytransmissive to certain wavelengths of interest such as 1.55 microns. Inaddition, processes for silicon fabricating such structures are welldeveloped. For these reasons, a waveguide comprising polysilicon andsilicon is advantageous.

[0089] Although silicon is beneficial because it is substantiallytransparent at certain wavelengths, other materials and moreparticularly, other semiconductors may be employed. Furthermore, thestructures described herein are not to be limited to any particularwavelength or wavelength range and may be designed, for example, formicrowave, infrared, visible, and ultraviolet wavelengths.

[0090] Those skilled in the art will appreciate that the methods anddesigns described above have additional applications and that therelevant applications are not limited to those specifically recitedabove. Also, the present invention may be embodied in other specificforms without departing from the essential characteristics as describedherein. The embodiments described above are to be considered in allrespects as illustrative only and not restrictive in any manner.

What is claimed is:
 1. A strip loaded waveguide comprising: a slabportion having a first refractive index (n₁), a strip portion having asecond refractive index (n₂), and a transition portion between the slabportion and the strip portion, the transition portion having a thirdrefractive index (n₃) that is less than the first refractive index (n₁)and the second refractive index (n₂).
 2. The waveguide of claim 1,wherein the transition portion comprises silicon dioxide.
 3. Thewaveguide of claim 1, wherein the slab portion comprises semiconductor.4. The waveguide of claim 3, wherein the slab portion comprises singlecrystal silicon.
 5. The waveguide of claim 1, wherein the strip portioncomprises polysilicon.
 6. The waveguide of claim 1, wherein the stripportion comprises single crystal silicon.
 7. The waveguide of claim 1,wherein the strip portion comprise silicon nitride.
 8. The waveguide ofclaim 1, wherein the slab portion and the strip portion comprisesubstantially the same material.
 9. The waveguide of claim 1, furthercomprising a substrate that supports said slab and said strip.
 10. Thewaveguide of claim 9, wherein the substrate comprises a silicon wafer.11. The waveguide of claim 9, wherein the substrate comprises sapphire.12. The waveguide of claim 1, further comprising a lower cladding layeradjacent said slab, said lower cladding layer having a refractive indexless than the first refractive index (n₁) of said slab.
 13. Thewaveguide of claim 12, wherein the difference between first refractiveindex (n₁) of said slab and said refraction index of said lower claddinglayer is at least about
 1. 14. The waveguide of claim 13, wherein thedifference between first refractive index (n₁) of said slab and saidrefraction index of said lower cladding layer is at least about
 2. 15.The waveguide of claim 12, wherein said lower cladding layer comprises alayer of silicon dioxide.
 16. The waveguide of claim 12, wherein saidlower cladding layer comprises a layer of silicon nitride.
 17. Thewaveguide of claim 12, further comprising an upper cladding adjacentsaid slab and said strip, said upper cladding has a refractive indexthat is less than both the first refractive index (n₁) and secondrefractive index (n₂).
 18. The waveguide of claim 17, wherein thedifference between a refractive index of said upper cladding and saidfirst refractive index (n₁) of said slab and said second refractiveindex (n₂) of said strip is at least about
 1. 19. The waveguide of claim18, wherein the difference between a refractive index of said uppercladding and said first refractive index (n₁) of said slab and saidsecond refractive index (n₂) of said strip is at least about
 2. 20. Thewaveguide of claim 17, wherein the upper cladding comprises glass. 21.The waveguide of claim 17, wherein the upper cladding comprises silicondioxide.
 22. The waveguide of claim 17, wherein the upper cladding andthe lower cladding comprise substantially the same material.
 23. Thewaveguide of claim 1, wherein said strip has a width and thickness, andsaid slab and transition regions have respective thicknesses so as tosupport a fundamental optical mode with a cross-sectional powerdistribution profile having two intensity maxima.
 24. A strip loadedsingle-mode waveguide for propagating light having a wavelength,comprising: a slab portion, a strip portion disposed with respect to theslab portion to form a guiding region, a first portion of the guidingregion being in the strip portion, and a second portion of the guidingregion being in the slab portion, the guiding region configured topropagate light of the wavelength only in a single spatial mode and onlyin a transverse electric mode.
 25. The strip loaded single-modewaveguide of claim 24, wherein said slab portion and strip portioncomprise single crystal silicon.
 26. The strip loaded single-modewaveguide of claim 25, wherein said slab portion has a thickness ofapproximately 90 nm and said strip portion has a thickness ofapproximately 40 nm.
 27. The strip loaded single-mode waveguide of claim24, further comprising a transition portion between the slab portion andthe strip portion, such that a third portion of the guiding region islocated in the transition portion.
 28. A strip loaded waveguidecomprising: a slab portion, a strip portion disposed with respect to theslab portion to form a guiding region, a first portion of the guidingregion being in the strip portion, and a second portion of the guidingregion being in the slab portion, the guiding region propagating lightin a single spatial mode with a cross-sectional power distributionprofile having two intensity maxima, one of which is located in the slabportion and the other of which is located in the strip portion.
 29. Thestrip loaded waveguide of claim 28, wherein the slab comprisessemiconductor.
 30. The strip loaded waveguide of claim 29, wherein theslab comprises silicon.
 31. The strip loaded waveguide of claim 30,wherein the strip comprises silicon.
 32. The strip loaded waveguide ofclaim 28, further comprising a transition portion positioned between theslab portion and the strip portion, such that a third portion of theguiding region is location in the transition portion.
 33. The striploaded waveguide of claim 32, wherein said transition portion comprisesoxide.
 34. A waveguide having a guiding region for guiding light throughthe waveguide, the guiding region comprising a layer of polycrystallinesilicon juxtaposed with a layer of crystal silicon.
 35. The waveguide ofclaim 34, wherein said layer of crystal silicon comprises a slab andsaid layer of polycrystalline silicon comprises a strip formed thereon,said strip and said slab forming a strip waveguide.
 36. The waveguide ofclaim 35, wherein said strip has a width and a thickness and said slabhas a thickness to support a single optical mode.